1. Field of the Invention
The present invention regards a device and a method for automatic calibration of a microelectromechanical structure included in a control loop.
In particular, the present invention finds an advantageous, but not exclusive, application in the compensation of the position offset of an inertial sensor, to which the ensuing treatment will explicitly refer, without this entailing any loss of generality.
2. Description of the Related Art
As is known, owing to their reduced size, excellent technical characteristics, high reliability and low cost, integrated inertial sensors manufactured using the micromachining technique are progressively laying claim to market segments up to now occupied by conventional inertial sensors.
One of the main applications of the above inertial sensors is in the field of airbag systems for motor vehicles as a means for measuring the deceleration to which a motor vehicle is subjected upon impact.
An inertial sensor, also known as accelerometric sensor or accelerometer, is a particular transducer device capable of measuring and converting an acceleration into an electrical signal, and is basically formed by two distinct elements: a sensor proper and an electrical interface.
The sensor proper is the element that is able to carry out conversion between the quantity (acceleration), the value of which is to be determined, and a quantity that may be measured by means of circuitry of an electrical nature, whilst the second element of the transducer device is a capacitive reading interface, i.e., a charge integrator, capable of determining the capacitance variation due to the presence of an acceleration.
An integrated rotary inertial sensor, i.e., the only movement of which is of a rotational nature, is described in the European Patent No. 99830568.4 filed on Sep. 10, 1999, in the name of the present applicant and is shown in FIG. 1.
The inertial sensor, designated as a whole by 1, is made of semiconductor material, has a circular structure, and comprises an inner stator 2 integral with the die 3 in which the inertial sensor 1 is formed, and an outer rotor 4 electrostatically coupled to the stator 2.
The rotor 4 comprises a suspended mass 6 having an annular shape, a plurality of mobile arms 8 extending radially towards the stator 2 from the suspended mass 6, identical to each other and angularly equispaced, and elastic-suspension and anchorage elements 10 (represented schematically as springs) elastically connecting the suspended mass 6 to fixed anchoring and biasing regions 12, through which the suspended mass 6 and the mobile arms 8 are biased (typically at a potential of 1.5 V).
The stator 2 comprises a plurality of pairs of fixed arms 14, 16, one pair for each mobile arm 8 of the rotor 4, which extend radially with respect to the suspended mass 6 towards the suspended mass 6 itself, are arranged in such a way that between each pair of fixed arms 14, 16 a corresponding mobile arm 8 of the rotor 4 is arranged, and are connected to respective fixed anchoring and biasing regions 18, 20, through which the fixed arms 14, 16 are biased (typically at a potential ranging between 1.5 and 2.2 V).
The fixed arms 14, 16 are connected, via the fixed anchoring and biasing regions 18, 20, to a sensing circuit having the purpose of picking up, from the fixed arms 14, 16, information regarding the relative position of the rotor 4 with respect to the stator 2.
The inertial sensor 1 can be electrically modeled as shown in FIG. 2, i.e., by means of two capacitive elements 21, 22 having a half-bridge configuration, wherein the two outer plates are defined by the fixed arms 14 and 16, respectively, of the stator 2, and the two inner plates are defined by the mobile arms 8 of the rotor 4.
When the suspended mass 6 is subjected to an angular acceleration, it undergoes a rotation such as to determine a modulation in phase opposition of the capacitances, indicated in FIGS. 2 as CS1 and CS2, of the capacitive elements 21 and 22, respectively, which, in the absence of angular acceleration or deceleration applied to the inertial sensor 1, should assume the same value. Consequently, by measuring the capacitances CS1 and CS2 it is possible to measure the magnitude of the unknown inertial quantity, i.e., the acceleration or deceleration to which the inertial sensor 1 is subjected.
On account of the imperfect configuration of the elastic-suspension and anchoring elements 10 and on account of the residual mechanical stress of the material of which the inertial sensor 1 is made, the rotor 4 is generally affected by a position offset, i.e., the effective zero position of the rotor 4 does not coincide with the nominal zero position, centered with respect to the stator, envisaged in the design phase.
The position offset consequently gives rise to a corresponding capacitive offset, defined as the difference between the capacitances of the capacitive elements 21, 22 in the absence of acceleration or deceleration, which has an adverse effect on the overall performance of the system comprising the inertial sensor 1 and the corresponding driving and measuring circuitry.
To carry out compensation of the aforesaid capacitive offset, the inertial sensor 1 is provided with an integrated microactuator 24 made of semiconductor material, coupled to the rotor 4 and having the purpose of rotating the rotor 4 by an amount equal to the position offset to bring it back into the nominal zero position.
In particular, the microactuator 24 comprises four distinct actuator groups 26, each of which is arranged in a respective quadrant of the inertial sensor 1 and is formed by a plurality of actuator elements 28, numbering four in the example illustrated in FIG. 1, identical to one another and angularly equispaced.
In detail, each actuator element 28 is defined on the silicon wafer together with the suspended mass 6 of the rotor 4, and comprises a mobile arm 30 integral with the suspended mass 6 (and consequently biased at the same potential as that of the suspended mass 6), extending radially outwards from the suspended mass 6, and carrying a plurality of mobile electrodes 32 extending from either side of the respective mobile arm 30 in a substantially circumferential direction, arranged parallel to one another, and equispaced along the respective mobile arm 30.
Each actuator element 28 further comprises a pair of fixed arms 34, 36 which extend radially with respect to the suspended mass 6, arranged on opposite sides of the corresponding mobile arm 30 and facing the latter, and connected to respective fixed anchoring and biasing regions 38, 40, through which the fixed arms 34, 36 are biased (typically at a potential ranging between 1.5 and 5 V). Each of the fixed arms 34, 36 carries a plurality of fixed electrodes 42, 43 extending in a substantially circumferential direction towards the corresponding mobile arm 30 and interleaved, or xe2x80x9ccomb-fingered,xe2x80x9d with the mobile electrodes 32 of the corresponding mobile arm 30.
The fixed arms 34, 36 of the actuator elements 28 are connected, through the fixed anchoring and biasing regions 38, 40, to a driving circuit (not shown) having the purpose of applying a biasing voltage to either one or the other of the two fixed arms 34, 36 of each actuator element 28 in such a way that the potential difference between the fixed arm 34, 36 thus biased and the corresponding mobile arm 30 causes a rotation of the rotor 4 in one direction or the other, sufficient for bringing the rotor 4 back into the nominal zero position.
In particular, as a result of the electrostatic coupling existing between each mobile arm 30 and the corresponding fixed arms 34, 36, the rotor 4 is subjected to a transverse force proportional to the number of pairs of fixed arms and mobile arms 30, 34, 36. This force tends to move the mobile arm 30 away from the fixed arm 34, 36, with respect to which the mobile arm 30 has a smaller potential difference, and to bring the mobile arm 30 closer to the fixed arm 34, 36, with respect to which the mobile arm 30 has a greater potential difference, thus causing rotation of the suspended mass 6.
Owing to the presence of the comb-fingered electrodes 32, 42, 43, the force necessary to bring the rotor 4 back from the effective zero position to the nominal zero position is altogether independent of the amount of offset with respect to the nominal zero position itself.
As regards implementation of the electrical interface of the inertial sensor 1, there essentially exist two different solutions.
The first solution consists in reading and amplifying the capacitance variation of the capacitive half-bridge of FIG. 2 caused by the angular acceleration to which the inertial sensor 1 is subjected. This technique represents a direct approach to the problem, whereby at output from the electrical interface there is a voltage directly proportional to the capacitance variation.
An alternative solution is proposed in xe2x80x9cA Three-Axis Micromachined Accelerometer with a CMOS Position-Sense Interface and Digital Offset-Trim Electronics,xe2x80x9d Mark Lemkin, Member IEEE, and Bernhard E. Boser, Member IEEE, IEE Journal of Solid-state Circuits, Vol. 34, No. 4, April 1999.
This solution basically consists in inserting the inertial sensor in a fedback system consisting of a control loop that measures the displacement of the rotor 4 with respect to its nominal position and accordingly applies to the rotor 4 a torque such as to maintain the rotor 4 in the nominal position. The value of the unknown inertial quantity (angular acceleration) is then proportional to the feedback torque necessary for nullifying the displacement of the rotor generated by the external load.
FIG. 3 illustrates the block diagram of the sensing device proposed in the above-mentioned publication.
The sensing device, designated as a whole by 50, comprises an inertial sensor 52 not provided with any actuator elements for compensation of the position offset, and a control loop 53 having the purpose of maintaining the rotor 4 in its nominal position and of measuring the acceleration to which the inertial sensor 52 is subjected.
From the control standpoint, the sensing device 50 has a circuit structure similar to that of a sigma-delta converter widely used in analog-to-digital conversion, wherein the inertial sensor 52 is inserted instead of the integrator (constituting the sigma part of the AD converter) and performs the conversion of the angular acceleration to which it is subjected into a variation in the capacitances CS1 and CS2 of the capacitive elements 21, 22.
The control loop 53 operates in a time division mode in the two sensing and actuation steps; namely, it switches between an actuation operating condition in which, through the fixed anchoring and biasing regions 18, 20 of the fixed arms 14, 16, it drives the rotor 4 to keep it in its nominal position, and a sensing operating condition in which, through the same fixed anchoring and biasing regions 18, 20, it measures the angular acceleration to which the inertial sensor 52 is subjected.
In particular, the control loop 53 comprises an adder 54 receiving at input an input acceleration xcex6IN, the value of which is to be measured, and a feedback acceleration xcex6RET supplied by a feedback branch which will be described hereinafter, and supplies at output an error acceleration xcex6ERR equal to the difference between the input acceleration xcex6IN and the feedback acceleration xcex6RET.
The error acceleration xcex6ERR is supplied at input to the inertial sensor 52, which supplies at output a capacitance variation xcex94CS indicative of the variation in the capacitances CS1 and CS2 of the capacitive elements 21, 22 of FIG. 2 caused by the error acceleration xcex6ERR, the capacitance variation xcex94CS being calculated according to the following relation:       Δ    ⁢          xe2x80x83        ⁢          C      S        =            ϵ      0        ·          S      gap      
where S is the area of the plates of the capacitive elements 21, 22, and gap is the variation in distance between the fixed arms 14, 16 of the stator 2 and the corresponding mobile arms 8 of the rotor 4 caused by the error acceleration xcex6ERR.
The control loop 53 further comprises a differential position interface 56 receiving at input the capacitance variation xcex94CS supplied by the inertial sensor 52 and supplying at output a voltage position signal VOUT indicative of the position of the rotor 4 and is calculated according to the following relation:       V    OUT    =                    Δ        ⁢                  xe2x80x83                ⁢                  C          S                            C        I              ·          V      M      
wherein CI and VM assume the meanings described hereinafter.
In particular, the position interface 56 is implemented using the differential circuit diagram shown in FIG. 4, i.e., using an operational amplifier 58 in fully differential configuration, the inverting and non-inverting input terminals of which are connected, through the capacitive elements 21, 22 in half-bridge configuration, to a voltage generator 60 supplying a square wave measurement voltage VM, and the inverting and non-inverting output terminals of which are respectively connected to the inverting and non-inverting input terminals via respective feedback capacitive elements 62, 64 having capacitance CI.
With reference again to FIG. 3, the control loop 53 further comprises a one-bit quantizer 66 receiving at input the position signal VOUT supplied by the position interface 56 and supplying at output a digital signal OUT assuming a first logic value, for instance 1, if the position signal VOUT is positive, and a second logic value, in the example considered 0, if the position signal VOUT is negative.
The digital signal OUT supplied by the one-bit quantizer 66 defines a sequence of bits generally referred to as xe2x80x9cbitstream,xe2x80x9d a term that will be used also in the ensuing treatment.
Finally, the control loop 53 comprises a main feedback branch 67 having the function of driving the rotor 4 to maintain it in its nominal position, and essentially formed by a main actuator 68 receiving at input the bitstream OUT supplied by the one-bit quantizer 66 and supplying at output the aforementioned feedback acceleration xcex6RET, which is indicative of the acceleration (and hence the torque) applied to the rotor 4 to maintain it in its nominal position, and the absolute value and sign of which define the intensity and direction of the feedback necessary for maintaining the rotor 4 in its nominal position.
In particular, the main actuator 68 acts directly on the biasing of the fixed arms 14, 16 of the stator 2, and consequently on the mobile arms 8 of the rotor 4, to maintain the rotor 4 in its nominal position, and is implemented using the differential circuit diagram shown in FIG. 5, in which the capacitive elements 21, 22 defined by the fixed and mobile arms 8, 14, 16 are shown.
In detail, the main actuator 68 basically comprises two pairs of switches, designated by 70, 72 and 74, 76, controlled by the bitstream OUT supplied by the one-bit quantizer 66.
The switches 70, 72 of the first pair are respectively controlled by the bitstream OUT and by the negated bitstream {overscore (OUT)} (obtained by means of a simple NOT logic gatexe2x80x94not shown) and connect selectively, and in phase opposition, the fixed arms 14 of the stator 2 to a supply line 78 set at a supply voltage VRET and to a ground line 80 set at a ground voltage VGND, whilst the switches 74, 76 of the second pair are also respectively controlled by the negated bitstream {overscore (OUT)} and by the bitstream OUT, and connect selectively, and in phase opposition, the fixed arms 16 of the stator 2 to the supply line 78 and to the ground line 80.
The biasing voltage actually applied to the fixed arms 14, 16 of the stator 2 thus comes to be a voltage correlated to the bitstream OUT, namely, a pulse modulated voltage modulated by the bitstream OUT (PDMxe2x80x94Pulse Density Modulation), and consequently also the feedback acceleration xcex6RET applied to the rotor 4 to keep it in its nominal position is correlated to the bitstream OUT.
The closed loop control of the position of the rotor of the inertial sensor 52 and, consequently, the measurement of the angular acceleration to which the inertial sensor 52 is subjected performed by the circuit structure of FIG. 3, are, however, adversely affected both by the position offset existing between the rotor 4 and the stator 2 and by the voltage offsets present in the active electronic devices, such as operational amplifiers and comparators, and by the mismatches of the passive electronic components, such as resistors and capacitors, that are present in the control loop 53.
In particular, owing to the factors referred to above, in the absence of angular acceleration applied to the inertial sensor 52, the bitstream OUT supplied by the one-bit quantizer 66 has a non-zero mean value, whereby, in this operating condition, the value of the feedback acceleration xcex6RET generated by the main actuator 68 is non-zero and thus causes, at best, a reduction in the intervention dynamics of the control loop 53, whilst, at worst, it may even lead to complete saturation of the control loop 53.
In fact, the control loop 53 is typically able to recover capacitance variations xcex94CS of the order of the fF, whereas the capacitance variations xcex94CS generated by the offsets and mismatches referred to above may reach values that are even decidedly higher. Consequently, in the presence of sufficiently small offsets and mismatches, the dynamics of the control loop 53 is reduced by the part necessary for recovering the said offsets and mismatches, whereas, in the presence of high offsets and mismatches, the control loop 53 is completely saturated, and its dynamics is consequently reduced to zero.
It has moreover been experimentally verified by the present applicant that the above-mentioned offsets and mismatches rarely assume sufficiently small values, such as to lead to a negligible, or in any case not too significant, reduction in the dynamics of the control loop 53; on the contrary, they often assume values such as to bring the control loop 53 to complete saturation. Consequently, the recovery of the aforementioned offsets and mismatches is increasingly becoming an indispensable requirement in this type of applications.
The disclosed embodiments of the present invention provide a sensing device and an automatic calibration method thereof that will enable the drawbacks described above to be overcome at least in part.
According to one embodiment of the invention, a sensing device is provided that includes a microelectromechanical structure made of semiconductor material, a control loop for controlling the microelectromechanical structure, the microelectromechanical structure including a stator element and a rotor element electrostatically coupled together, and the control loop including an interface circuit coupled to the microelectromechanical structure and supplying a position signal indicative of the position of the rotor element; and a calibration circuit for calibrating the structure, the calibration circuit including at least one actuator made of semiconductor material and coupled to the rotary element, the first driving circuit for driving the at least one actuator including a first driving circuit receiving a position signal and supplying to the at least one actuator a driving signal correlated to a mean value of the position signal.
In accordance with another embodiment of the invention, a method for automatic calibration of a sensing device is provided, the sensing device including a microelectromechanical structure made of semiconductor material and a control loop for controlling the same, a microelectromechanical structure including a stator element and a rotor element electrostatically coupled together, the control loop including an interface circuit coupled to the microelectromechanical structure and configured to supply a position signal indicative of the position of the rotor element. The method includes the step of repositioning the rotor element to overcome voltage offset and component mismatches, which further includes the steps of providing an actuator made of semiconductor material and coupled to the rotor element; and supplying to the actuator a driving signal correlated to a mean value of the position signal.
In accordance with another aspect of the foregoing method of the present invention, the control loop includes a quantizer for receiving the position signal and supplying a corresponding sequence of samples, and the step of driving the actuator includes the step of supplying to the actuator a driving signal correlated to a mean value of the sequence of samples.